This invention relates to a method for repairing a mold for continuous casting of steel.
Generally speaking, the mold for continuous casting is made of copper or copper-alloy body with a nickel or nickel-base alloy layer plated thereon. Sometimes a chromium layer may additionally be plated on said plated layer. When the surface plated layer is worn and scratched in use, it is usually subjected to a repairing operation.
The conventional way of repairing was to completely abrade and remove all the nickel or nickel-base alloy layer from the surface and thereafter apply a new surface thereon.
FIG. 1 shows an example where a nickel or nickel-base alloy layer 2 and a chromium layer 3 have been applied to the surface of a mold body 1 made of copper or copper alloy. When it is desired to repair it, the abrading operation is made to the extent of a plane A--A of FIG. 1 to remove the entire plated layer and thereafter form a new plated layer thereon. It is because there has been an insufficient adhesion problem between the two metals of the same kind plated on each other and a degradation problem in use of the nickel or nickel-base alloy plated layer.
The thickness of the nickel or nickel-base alloy plated layer applied to the lower surface of the mold body tends to become greater nowadays as the speed of the continuous casting becomes faster. The abrading of the surface of a mold has been conducted independent of the degradation of its plated layer and, therefore, it is obviously undesirable, from the view point of the mold costs and the material savings, to remove all parts of the layer including a part which can fully be reused.
Under the circumstances, the inventors have made attempts not to remove all the expensive nickel or nickel-base alloy layer but to leave a part of it on the mold body, and to apply thereon a new nickel or nickel-base alloy layer according to the known method. This is exemplified in FIG. 1 where the abrading operation is conducted to the line B--B of the nickel layer 2, on which a new nickel layer is plated. However, the adhesion power by shearing between the basic old plated layer and the newly provided layer is 5 to 25 kg/mm.sup.2, which shows considerable fluctuation. It is thus clear that the molds repaired according to the above include one which can never be practically used at such high temperature and under such severe conditions as applied to the mold for continuous casting of steel. According to the inventors' study, the adhesion power between the new and old plated layers in the mold for continuous casting of steel necessitates at least 20 kg/mm.sup.2 and the fluctuation in this case must be .sigma..sub.n .ltoreq.3.0 where .sigma..sub.n is the standard deviation. This means that so long as the known method is followed, there is a tendency that many materials may be dipped, which must be improved.